The present invention relates to an optical inspection method and an optical inspection apparatus for inspecting a minute defect on the surface of a sample by irradiating the sample with light and detecting reflected light therefrom.
In the manufacturing lines of semiconductor substrates, thin film substrates and the like, optical inspection apparatuses are used extensively to inspect minute defects on the surface of samples so as to acquire a high product yield (e.g., JP-9 (1997)-304289-A (Patent Literature 1) and JP-2006-201179-A (Patent Literature 2)). Generally, the optical inspection apparatus irradiates the sample surface with a focused light beam of several tens of μm across, detects transmitted light, scattered light, or reflected light from any defect, and focuses the detected light for defect detection. Currently commercialized apparatuses can inspect defects of several tens of nm or more across.
Meanwhile, with advances in fine processing technology, there has been a growing need for inspecting defects more minute than ever. Because a minute defect gives off faintly reflected light when irradiated with illumination light, more sensitive, higher-performing defect detection technology is needed. In particular, given faint light, it is impossible to ignore the influence of fluctuation called quantum noise stemming unavoidably from the uncertainty principle of quantum mechanics. This makes it important to suppress the influence of quantum noise.
One method for detecting such minute defects, as described in JP-2007-232667-A (Patent Literature 3) for example, involves identifying the presence or absence of a defect based on information about the differences in phase and amplitude between the light obtained from the sample typically through homodyne detection or heterodyne detection on the one hand, and reference light on the other hand.